Aberrant activations of the STAT3 (signal transducer and activator of transcription 3) signaling pathway are associated with cancer and inflammatory diseases. Three of the four Janus kinases, JAK1, JAK2, and Tyk2, are the major upstream kinases of STAT3 in responses to cytokine stimulations. Among them, JAK2 is the key kinase in the IL-6-induced STAT3 phosphorylation. Here we report the mechanisms of a natural compound parthenolide from the medicinal herb Feverfew in regulating the JAK/STAT3 signaling. We found that parthenolide was a potent inhibitor of JAKs. It covalently modified the Cys178, Cys243, Cys335, and Cys480 of JAK2 and suppressed its kinase activity. It also interacted with other JAKs in a similar fashion. The binding of parthenolide to JAKs was selective. It preferentially bound to the JAKs, but not to the abundant proteins, such as tubulin and actin. Parthenolide also induced reactive oxygen species (ROS), but the increased ROS did not seem to contribute to the inhibition of JAK/STAT3 signaling. Furthermore, parthenolide inhibited the IL-6-induced cancer cell migration and preferentially inhibited the growth of cancer cells that had constitutively activated STAT3. Our study suggests a novel strategy to inactivate JAKs and provides a promising anti-inflammation and anticancer drug candidate.
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